Digital display systems typically produce or modulate light as a linear function of input image data for each pixel. For an 8-bit monochromatic image data word, the input image data word ranges from 0 to 255. A value of 0 results in no light being transmitted to or produced by a pixel, 255 is the maximum intensity level for a pixel, and 128 is mid-scale light.
Pulse width modulation (PWM) schemes typically modulate a constant intensity light source in periods whose length increases by a power of two. For example, when 5 mS is available for each color of a three-color system the element on times for one 8-bit system are 20 μS, 40 μS, 80 μS, 160 μS, 320 μS, 640 μS, 1280 μS, and 2560 μS. If a given bit for a particular pixel is a logic 0, no light is transmitted to or generated by the pixel. If the bit is a logic 1, then the maximum amount of light is transmitted to or generated by the pixel during the bit period. The viewer's eye integrates the light received by a particular pixel during an entire frame period to produce the perception of an intermediate intensity level.
One problem created by PWM schemes is the creation of visual artifacts that arise due to the generation of an image as a series of discrete bursts of light. While stationary viewers perceive stationary objects as having the correct intensity, motion of the viewer's eye or motion in the image can create an artifact know as PWM temporal contouring, or simply PWM contouring. PWM contouring is described in U.S. Pat. No. 5,619,228 and occurs when the distribution of radiant energy during a frame period changes from one frame to the next. With motion in the scene or eye, portions of the frame time of adjacent image pixels are integrated to achieve an incorrect perceived pixel intensity. The PWM temporal contouring artifact appears as a noticeable pulsation in the image pixels. This pulsation is time-varying and creates apparent contours in an image that do not exist in the input image data.
PWM contouring is most clearly seen when viewing a grayscale ramp that increases horizontally across an image. As the image data on each line increase from 0 on the left of the row to 255 on the right, there are several places along each row where the major bits change from a logic 0 to a logic 1. The most dramatic change is in the center of each row where one pixel has a binary value of 127, which results in the first seven bits being a logic 1, and the adjacent pixel to the right having a binary value of 128, which results in the first seven bits being a logic 0 and the most significant bit being a logic 1.
If the image data is displayed over time in order of decreasing bit magnitude, that is b7, b6, b5, b4, b3, b2, b1, and b0, a viewer scanning from left to right may see an abnormally bright region at the 127 to 128 transition. This abnormal brightness is due to the viewer's eye integrating the last half of a given frame of pixel data 127—during which all bits 6:0 are all on—with the first half of the next frame—during which bit 7 is on for the entire half-frame. The net effect of the integration of the last half of the 127-valued pixel and the first half of the 128-valued pixel is a pixel having an intensity value of 255. The same artifact occurs when the pixel data is moving and the viewer's eye is stationary, and at the lower bit transitions.
When viewed at a normal viewing distance, the PWM contouring artifact created by two adjacent pixels is very difficult, if not impossible, for the typical viewer to detect. In real images, however, the bit transitions often occur in areas having a large number of adjacent pixels with virtually identical image data values. If these large areas of similar pixels have clusters whose intensity values cross a major bit transition, the PWM contouring is much easier to detect.
One method of reducing the PWM contouring artifact uses bit splitting. Bit splitting divides the long periods during which the more significant bits are displayed into two or more shorter bits and distributes them throughout the frame period. For example, an 8-bit system may divide the MSB, having a duration of 128 LSB periods, into four equal periods each requiring 32 LSB periods and distributed throughout the frame period.
Bit splitting techniques eliminate most of the objectionable PWM contouring artifacts. Unfortunately, bit splitting increases the necessary bandwidth of the modulator input since some of the data must be loaded into the system multiple times during a single frame period. Practical bit splitting methods reduce, but do not eliminate, the PWM contouring artifact.
One solution to the PWM contouring problem, called boundary dispersion, dithers the image data for each pixel in or near a region that crosses such a major bit boundary. As described in U.S. patent application Ser. No. 09/088,674, spatial patterns are used to dither pixels above and below the values at which the major bit transitions occur. For example, if a major bit transition occurs at an intensity value of 32, all pixels within a certain distance of a pixel having a value equal to 32, or a value in a range including 32, are dithered. In one embodiment, a +/−2 LSB dither is applied in a checkerboard fashion over a repeating two-frame period. During the first frame, the intensity of alternate pixels is reduced by 2 LSBs while the intensity of the other half of the pixels is increased by 2 LSBs. During a second frame, the dither is reversed. Over the two frame period, each pixel is displayed at the correct average intensity. The size of the dither varies as a function of intensity data values of the pixel and nearby pixels.
Boundary dispersion spreads the region in which the major bit transition occurs over a larger area and moves the actual transition each frame. Unfortunately, boundary dispersion creates spatial noise patterns within any given frame and temporal noise from frame-to-frame. What is needed is a system and method for reducing the PWM contouring artifact as well as other artifacts, that does not induce spatial and temporal noise or other artifacts into the resulting image.